Active silicon photonic devices play critical roles in future silicon photonic integrated circuits including light emitting devices, optical modulators and detectors. Generally speaking, in order to integrate active functions into a passive device system, it is necessary to dope the silicon waveguide with either donor or acceptor ions. In this process, the introduced free carriers induce unavoidable absorption loss under optical communication wavelength following the free carrier's effect which degrades the performance of photonic devices. In traditional p-i-n silicon modulator with larger rib waveguide dimensions, this problem is not that distinct especially after enlarging the distance between doping regions and optical waveguide. However, when the modulator's dimension becomes smaller (450×250 nm2), diffusion of free carriers into non-intentional doped region has to be considered.
How to reduce the absorption loss coming from the diffused carriers has been one of the most urgent problems. For example, the recently reported silicon optical modulator with the highest switching speed is realized with a reversed bias PN junction lying near the centre of the waveguide [J. Basak, et al., Adv. Opt. Technol., vol. 2008, 678948-1-678948-10 (2008)]. With a useful extinction ratio (˜6.1 dB), the switching speed reached 10 Gb/s with 2 mm-long phase shifter and the corresponding loss caused by two arms is 1 dB/mm [T. Y. Liow, et al., IEEE J. Sel. Top. Quantum. Electronics, 16, 307-315 (2010)]. Another reported 1 mm-long modulator can operate at 12 GHz modulation speed with a 6 dB extinction ratio under −8 V bias with 2.5 dB insertion loss [N. N. Feng, et al., Opt. Express, 18, 7994-7999 (2010)]. The high additional loss mainly comes from the free carriers' absorption in the doped regions [N. N. Feng, et al., Opt. Express, 18, 7994-7999 (2010)].
A p-i-p-i-n diode has also been presented to achieve low loss optical modulator with the same cutoff frequency as that of a pn diode [D. M. Morini, et al., Opt. Express, 16, 334-339 (2008); G. Rasigade, et al., IEEE J. Quantum Electronics, 16, 179-184 (2010)]. However, both of the intrinsic regions are affected by the lateral diffused electrons and holes from neighboring p-doped and n-doped regions. For boron ions implanted with 80 keV ion energy, the lateral diffused length reaches 75 nm [U. Littmark, and J. F. Ziegler, Phys. Rev. A., 23, 64-72 (1981)] which made the neighbor intrinsic region actually a p-type semiconductor.
The performance of a silicon optical modulator depends on three main parameters, such as the modulation efficiency, the switching speed and the total loss. Presently, the loss of a conventional silicon optical modulator is too high for commercial use. In the market for optical communication, the insertion loss of 10 Gbps Lithium Niobate modulator is nearly 4-5 dB. If the absorption loss of the silicon modulator can be reduced, the performance of the silicon modulator may be comparable to that of the Lithium Niobate modulator. However, current approaches for the reduction of the total loss always comes with the degradation of other characteristics, including the modulation efficiency and/or the switching speed.